How to Read Diagnostic Trouble Codes - Breaking The Code

For all its '81 vehicles, GM introduced a computer-controlled engine-management system named Computer Command Control. It became known as CCC or C-3.

The engine controller—first called an engine control module (ECM), and then later called a powertrain control module (PCM) when the electronically controlled transmission was added—had the ability to monitor itself and identify a wide range of problems if one occurred under certain conditions and for a specific amount of time. It stored a defined Diagnostic Trouble Code (DTC) in its computer memory and evoked an amber dashboard light. The ability to self-diagnose was often a confounding aspect of the GM system. In the beginning, only a few people took the effort to understand it.

Breaking the code

The ECM is mounted in the passenger compartment, while the later PCM is usually found under the hood. The system was eventually identified as OBD-I for Onboard Diagnostics, First- Generation. (The requirements of an OBD-I system were set forth by California Air Resources Board in an effort to improve the air quality in the state of California beginning in model-year '88 vehicles.)

Underneath the dash on '81-'95 Pontiacs is an Assembly Line Diagnostic Link (ALDL) connector. For most of these model years, it has 12 pins. (Note: The '81 version had 5 pins, and the '95s we have seen have a 16-pin connector.) A scanner plugs into this connector.

GM designed these pre-OBD-II systems to allow mechanics to read trouble codes without a scan tool. All they had to do was turn the ignition key on without the vehicle running and jump the two righthand ALDL terminals (A to B; see caption 6), forcing the ECM into a diagnostic mode. (These days, companies such as Actron sell code readers that you simply plug into the ALDL and it displays the code for you.)

Once in diagnostic mode, the computer flashes the Check Engine or Service Engine Soon light (the specific name varies by year) in a fashion similar to Morse code. Each code is a series of flashes followed by a pause, and then a second series of quick flashes. (Note: When this diagnostic mode is evoked, the first code is always 12—one flash and then two quick flashes. This code signifies no distributor reference signal, and, since the vehicle is not running at this time, this is correct. The reason code 12 is prioritized first in the sequence is because GM engineers wanted the system to display that it was operational.) Here's another example: flash, flash, pause, flash, flash, flash—that's code 23.

Each flashing sequence displays three times per code, and then the next higher numerical code displays. When all the stored codes have been shown, the sequence, starting with code 12, begins again. To exit diagnostic mode, simply remove the scanner or jumpers.

If the system does not flash code 12, the vehicle's shop manual will have a diagnostic routine of troubleshooting steps, called Will Not Flash Code 12 or something similar. The most common failures if the vehicle will not go into diagnostic mode is a faulty engine controller (see caption 7) or a ground problem going to the ALDL.

The ECM has two voltage sources. One is constant 12 volts and the other is switched on with the ignition key. The memory where the trouble codes are stored is connected to the constant feed. On '81-'95 Pontiacs, if the battery is disconnected or goes dead, then the stored trouble codes are erased. Therefore you can purposefully erase a trouble code and clear the adaptive memory by disconnecting the battery cable from the negative terminal for ten seconds.

On occasion, a code can be set from false data, electrical interference, or a momentary poor-running condition. (This is a common problem on OBD-I vehicles.) It is prudent for a mechanic to erase codes, operate the vehicle, and then see what codes come back. If a code returns, underlying problem is likely a valid one.

If you invest in a scan tool, then a wealth of data beyond identifying DTCs is available

Two common mistakes that both professional and amateur mechanics must guard against is to presume that a stored trouble code means the sensor for that circuit is defective, and then conclude the sensor is the failed part. GM made it very clear that these codes are circuit codes and not sensor codes. For example, Code 13 means the oxygen-sensor circuit does not see enough activity. The problem could be the sensor, the wiring/connection, a vacuum leak or leaks, or the carburetor.

To reiterate, mechanics should guard against simply looking up the trouble code in a shop manual and replacing the sensor that is referenced, only to find out that this did not repair the problem. A good rule is if the system is loaded with codes, then the problem is usually a ground problem. If you suspect a problem with the ECM, tap on the ECM gently with your fist and note for any changes in engine performance.

There is a small electronic chip on board the ECM called a Programmable Read-Only Memory (PROM). Hard-burned on this chip are the instructions that run the engine, such as mixture control, ignition timing, and spark advance. GM installed a hierarchy within the on-board diagnostic procedure. It usually states that any code that begins with a five needs to be addressed first since it deals with the ECM and/or PROM.

If you invest in a scan tool, then a wealth of data beyond identifying DTCs is available. Many scan tools feature a data display, which allows you to visually monitor the conversation the engine is having with the ECM, including ignition advance, fuel control, torque converter clutch activation, and a myriad of other details.

OBD-II was introduced in '96 model-year vehicles, and we will make it the subject of a future tech story. For now, we'll point out two casualties of this Generation-Two system: the ability to flash stored trouble codes, and the method to erase stored trouble codes by disconnecting the vehicle battery for a short period of time.

Most auto-part stores loan code readers that you can use on-site to view the DTCs that evoked the Check Engine light on your dash, but you may find that they don't carry the cable necessary to connect to the ALDL in your '81-'95 Pontiac. That's why we found that owning an inexpensive code reader, such as the Actron CP9001 (MSRP $30), may be a wise investment for your toolbox.

As a service to our readers, we've listed many of the common Diagnostic Trouble Codes for '81-'95 Pontiacs in a chart on the preceding page.

Note: Due to the once-emerging science of onboard self-diagnostics as it applied to '81-'95 Pontiacs, the information in this article is deemed generally accurate, and is intended as a primer only. You should consult your vehicle's Pontiac Service Manual for clarification, if needed.

Special thanks to James Berry and General Motors' Daniel P. Grenn for their help with this story.

2/91. All ’81-’95 Pontiacs feature a pre-OBD-II Assembly Line Diagnostic Link (ALDL) connector that allows you to read stored Diagnostic Trouble Codes (DTCs). The OBD-I compliant ALDL on this ’88 Firebird is below the dash and behind this panel. Model-year ’96-and-newer Pontiacs use an OBD-II protocol.

3/92. Opening the panel reveals the ALDL 12-pin connector. (All ’81 and some, if not, all ’95 Pontiacs use a different connector.) Some Pontiacs do not have a cover over the ALDL.

4/93. Actron’s Code Scanner (PN CP9001, MSRP $29.95) makes it easy to read the flash-code sequence GM used prior to implementing OBD-II. Just plug it into your Pontiac’s ALDL connector and decode the flashes. (We explain how in the story.)

5/94. For Pontiac owners looking to monitor the myriad types of data available on the OBD-I data bus, Actron’s Elite AutoScanner (PN CP9185, MSRP $249.95) is a full-featured code reader/scanner, compatible with both OBD-I and OBD-II.

6/95. Making it work on an ’88-’94 Pontiac requires Actron’s GM OBD I ALDL cable (PN CP9127, MSRP $29.99).

7/96. This diagram shows the pin outs of the 12-pin OBD-I connector. Note that pins A (ECM Ground) and B (Diagnostic Request) are the farthest right on the upper row.

8/97. Here’s an example of an Engine Controller Module, or ECM.

9/98. In this example, the PROM was removed from its socket. When you hear the phrase “burn the chip,” it’s referring to the PROM.

Common '81-'95 diagnostic trouble codes
Note: Not all codes apply to all years and models